It is generally acknowledged that years of industry have generated numerous environmentally tainted sites throughout the country and around the world, which pose health hazards to people. In recent years, efforts to clean up or remediate environmentally contaminated sites have increased dramatically. Many methods and devices for cleaning up or disposing of environmental contamination in water, air, and soil have been developed. However, the magnitude of the environmental contamination remains vast in comparison to the resources made available to solve this growing problem.
Soil contamination is a matter of concern in many locations. The ever increasing environmental awareness has produced more stringent laws and regulations for the protection of human and animal health. At least in the developed countries it is no longer tolerated for contaminated soil to be deposited in open landfills, while special deposit sites for hazardous waste become more limited and increasingly expensive.
“Soil” refers to unconsolidated and consolidated material in the ground. Soil may include natural formation material such as dirt, sand, minerals, ores, geological deposits, and rock, as well as fill material. Soil may be contaminated with chemical, biological, and/or radioactive compounds. Contamination of soil may occur in a variety of ways, such as material spillage, leakage from storage vessels, and landfill seepage. Public health concerns may arise if such contaminants migrate into aquifers or into air. Soil contaminants may also migrate into the food supply through bioaccumulation in various species part of the food chain.
To address the problem of environmental contamination and particularly as it relates to soil contamination, a range of soil treatment and decontamination techniques have been developed. These techniques involve, but are not limited to the application of fluids, biological agents, heat, vacuum, pressurized gases, and mechanical agitation. There are many ways to remediate contaminated soil. “Remediating soil” means treating the soil to reduce contaminant levels within the soil or to remove contaminants from the soil. An ex situs method of remediating contaminated soil is to excavate the soil and then process the soil in a separate treatment facility to reduce contaminant levels within the soil or to remove contaminants from the soil. Alternatively, contaminated soil may be remediated in situs.
In situs flushing operations wash away contaminated areas in the soil or in the groundwater with large amounts of water (provided the flushing water can be isolated and separated efficiently). Instead of using water, it is also possible to separate volatile organic substances from soil with (hot) steam. However, this requires an after-treatment of the extracted materials.
Washing of contaminated soil is conventional. Water can cleanse contaminating substances from soil, gravel and rocks mechanically. Water also may separate fine-grained particles (silt and clay) from coarse-grained particles (sand and gravel). In this case the water is loaded with the contaminants from the soil and needs to be treated adequately, before it can be reused or returned to surface and ground water.
One typical site of environmental soil contamination involves weapons firing ranges (e.g. small arms fire) where heavy metal contamination is a serious on-going problem. Such contamination poses substantial economic costs and environmental challenges to surrounding areas. Remediation of soil from firing ranges poses special concerns because the contaminants typically exist both as discrete particles (unfired rounds, intact expended bullets, bullet fragments and finer-sized heavy metal particles) and ionic (adsorbed) heavy metals residing on the soil grains. The ionic coatings are often the result of corrosion of the particulate lead, which can leach into the ground water and cause serious environmental and human health consequences.
Small arms firing ranges accumulate particulate lead from expended bullets. Depending on site-specific characteristics such as soil type, exposure time, and rainfall, erosion and migration of lead may occur. Soil outside of the range as well as ground water may become contaminated with lead that has corroded, dissolved, and migrated from the expended rounds. As a consequence of the heavy metal content, firing range soils typically fail Toxicity Characteristic Leaching Procedure (TCLP) tests, thus mandating that the large volume of material on such sites must be managed as hazardous waste. Current remediation strategies do not comprehensively and economically address the problem of both particulate and leachable (e.g., ionic heavy metal) soil contamination under all climatic and soil conditions.
Two distinct types of treatment goals may be applied for firing range soils: total content and leachable content. The leachable (hazardous) content is related to the total content, but is not directly proportional. The leachability of a metal can be strongly influenced by the physical and chemical conditions of the metal.
To date, no technology has been available that can simultaneously reduce both total and leachable contaminant concentrations (e.g., lead) and render soil acceptable for reuse. Soil leachability can be mitigated using stabilization; this technology has a proven track record and is available from numerous vendors. Stabilization does not remove lead, leaving the total lead content unaffected. As a result, stabilized soils tend to contain high total lead contents and must still be disposed of off-site.
Therefore, there is a need for developing a process that provides better, more rapid and less expensive environmental clean up of contaminated sites and materials. There is a further need for developing a process which has not previously been available that removes contaminants such as heavy metals to lower the total soil contaminant content as well as the leachable contaminant content. There is a further need for developing a process that can treat contaminated soil, including small arms firing range soil to meet both remediation criteria, thus enabling soil reuse instead of resorting to costly and unproductive landfill disposal.